This invention relates to processing of fuel feedstocks containing hydrocarbons for use in fuel cell systems and, in particular, to pre-processing assemblies for performing pre-processing of the fuel feedstocks.
A fuel cell is a device which directly converts chemical energy stored in hydrocarbon fuel into electrical energy by means of an electrochemical reaction. Generally, a fuel cell comprises an anode and a cathode separated by an electrolyte, which serves to conduct electrically charged ions. Molten carbonate fuel cells operate by passing a reactant fuel gas through the anode, while oxidizing gas is passed through the cathode. In order to produce a useful power level, a number of individual fuel cells are stacked in series with an electrically conductive separator plate between each cell.
Current fuel cells require as the reactant fuel gas a clean gas composed of hydrogen or a mixture of hydrogen and carbon monoxide. The reactant fuel gas is generally developed from a hydrocarbon-containing feedstock using a reforming process. The hydrocarbon-containing feedstock usually contains substantial amounts of lower hydrocarbons, i.e., hydrocarbons with 2 or less carbons, such as methane, as well as small amounts of hydrogen, carbon dioxide, nitrogen and higher hydrocarbons, i.e. hydrocarbons with more than 2 carbons. This is true, for example, when the fuel feedstock is natural gas, peak shaving gas, digester gas and coal bed methane.
The fuel feedstock is usually subjected to pre-processing to reduce or eliminate the higher hydrocarbons and to convert a portion of the lower hydrocarbons to methane, hydrogen and carbon dioxide. The feedstock is then further processed in a reforming unit to generate a fuel gas rich in hydrogen.
Conventional pre-processing is carried out using a deoxidizer assembly followed by a pre-reforming assembly. The deoxidizer assembly reduces the concentration of oxygen in the fuel feedstock before the feedstock enters the pre-reforming assembly. This protects the catalyst (usually, a Ni-based catalyst) used in the pre-reforming assembly, which otherwise would be deactivated in the presence of oxygen.
In the pre-reforming assembly, the reforming reaction is a conversion process which may inadvertently result in carbon formation based on fuel composition and steam. Carbon formation is of a particular concern when the fuel feedstock contains propylene, since the propensity to form carbon increases as the concentration of propylene increases. The carbon which is produced deposits at the active sites of the reforming catalyst of the pre-reforming assembly, thereby deactivating the catalyst. This reduces the life of the pre-reforming assembly.
In order to reduce carbon formation in conventional pre-reforming assemblies, special catalysts either containing alkali or based on an active magnesia support have been proposed. Another technique is to use adiabatic processing. In such case, a fixed bed adiabatic pre-reforming assembly converts the higher hydrocarbon content at low temperature with steam into methane, hydrogen and carbon oxides.
Propylene-containing fuel feedstocks generally have a high concentration of sulfur-containing compounds. These compounds also tend to deactivate the reforming catalysts in the pre-reforming assembly. Although fuel feedstocks are typically desulfurized in a desulfurizer unit before being carried to the pre-reforming assembly, high sulfur concentration and the propylene in the fuel feedstocks reduce the capacity of the desulfurizer unit.
Fuel feedstocks supplied to the pre-reforming assembly must also be supplied with additional hydrogen from a hydrogen supply. This is required to provide a sufficient concentration of hydrogen in the feedstocks to maintain a reducing environment for the reforming catalyst, thereby maintaining the catalyst activity.
As can be appreciated, conventional pre-processing of fuel feedstocks is complex and costly, due to the need for additional units or special components for supplying hydrogen, for reducing carbon formation and for removing propylene and an additional unit to remove oxygen entering into the pre-reforming assembly. A pre-processing assembly of simpler design, less cost and longer life would thus be desirable.
It is therefore an object of the present invention to provide a pre-processing assembly which is better able to process fuel feedstocks containing hydrocarbons and oxygen without deactivation of the pre-processing catalyst.
It is a further object of the invention to provide a pre-processing assembly which is capable of operating without an additional hydrogen supply and has an increased operating life.
It is yet a further object of the invention to provide a pre-processing assembly which is specifically adapted to retard the affects of propylene and other olefins in hydrocarbon containing fuel feedstocks.